Eddy current sensors are known and used in a variety of applications. One use of eddy current sensors is monitoring for defects in turbine blades of a turbine engine. More particularly, as a turbine blade spins in the turbine engine, the blade is affected by centrifugal forces. The constant centrifugal forces along with vibration may induce material defects in the turbine blade. As the material defects grow, the length of the turbine blade may increase incrementally and eventually meet the inner diameter of the turbine casing and thus cause damage to the turbine engine.
U.S. Pat. No. 5,942,893 (issued Aug. 24, 1999) to Terry (“'893 patent”) discloses eddy current sensors for detecting various parameters, e.g. Blade tip clearance, speed and time of arrival of rotating blades of turbo machinery. The detected information is used for monitoring the performance and condition of the machinery.
The eddy current sensors disclosed in the '893 patent (the subject matter of which is incorporated herein by reference) comprise a generally E-shaped core having three parallel legs joined together by a bridge. The two outer legs include oppositely wound coils generating two high frequency magnetic fields which combine to form the sensing magnetic field. A third sensing coil is wound around the center leg of the E-shaped core and is connected to a signal processing circuit.
In the absence of any moving electrically conductive object within the magnetic sensing field produced by the two outer coils, the sensing field remains undisturbed and no voltages or signals are produced in the sensing coil. However, when an electrically conductive object, e.g., the rotating blade of a turbine, passes through the magnetic field, eddy currents are generated within the conductive object. The eddy currents themselves generate magnetic fields, and, as these eddy current produced magnetic fields interact with the sensing field, disturbances occur in the sensing field which induce signal voltages in the sensing coil on the center leg of the E-shaped core. The induced signal in the sensing coil, once demodulated, generates a waveform signal with negative and positive peaks and a zero crossing point.
Analysis of the induced coil signal, as described in the '893 patent, provides various information about the moving object, e.g., the speed of the object, its standoff distance from the sensor and the time of its passage by the two outer legs and the center leg of the sensor. As described in the patent, such information is useful for monitoring the operating characteristics of turbo machinery. As discussed above, the two outer coils are the drive coils producing the magnetic field. The two drive coils, however, do not necessarily need to be positioned on the outside of the eddy current sensor and may be positioned on the center coil with one of the outer coils being the sensing coil.
The traditional approach to the analysis of the coil signal, and particularly the standoff distance, includes calculating the standoff distance as a function of the peak-to-peak voltage between the negative and positive peaks of the demodulated waveform. In a properly designed eddy current sensor this voltage does not depend on rotating blades temperature. Unfortunately, magnitude of the waveform and this peak-to-peak voltage are sensitive to temperature and dependent on the temperature of the eddy current sensor and sensor electronic circuits. As such, additional temperature sensors and additional electronics are also necessary to measure the temperature of the sensors and sensor electronics in order to compensate for temperature variations. Temperature compensation algorithms and data also require significant processing capability to manage computing standoff distance as a function of both peak-to-peak voltage and the temperature of the sensor and sensor electronics.
Therefore, there exists a need for a method of computing standoff distance that is independent of the temperature of the eddy current sensor and the sensor electronics.